Method and apparatus for detecting electrostatic charges during semiconductor fabrication process

ABSTRACT

A process for detecting electrostatic charges on a wafer surface during the fabrication of semiconductor devices after the wafer is subject to a de-ionized water rinsing step to remove particles and other impurities from a wafer surface. This process includes the steps of: (a) positioning an insulation layer above a wafer surface on which electrostatic charge densities are to be scanned; (b) using a movable probe to measure voltages at various locations at the insulating layer; (c) collecting the measured voltage distribution; and (d) examining the collected voltage distribution to identify areas on the wafer surface correspondingly to high electrostatic charge density. Because high precision and high resolution scanning can be made with this method, trouble-shooting diagnosis to be expeditiously performed with minimum interruptions to the semiconductor fabrication operation.

FIELD OF THE INVENTION

The present invention relates to a method which can be very used todetect electrostatic charges following a de-ionized water rinsing stepduring the fabrication of semiconductor devices to remove particles andother impurities from a wafer surface. More specifically, the presentinvention relates to a novel method, and an apparatus for carrying outthe same, for use in the semiconductor industry, to conveniently andcost-effectively detect electrostatic charges that may be present on thesurface of a semiconductor wafer following a rinsing step which usespure water or de-ionized water, to remove particles, and/or otherimpurities. One of the most distinct advantages of the method of thepresent invention is that, because of its precision and high resolution,it can very quickly and accurately identify the troubled spot so as toallow a trouble-shooting diagnosis to be expeditiously performed.Another main advantage of the present invention is that, because of therelatively simple procedure involved, it can be very cost-effectivelyimplemented which will cause only minimum interruptions of thefabrication operation.

BACKGROUND OF THE INVENTION

Rinsing is a very important step in semiconductor fabrication processes.Typically, the rinsing step is performed using ultra high quality purewater or de-ionized water. Conventionally, the effectiveness of arinsing process is measured by detecting the presence of particles orsurface impurities, or by forming a metal-oxide semiconductor (MOS)device on the original oxide layer to qualitatively measure thestress-resistance thereof.

If the rinsing water contains particles or other impurities, theseforeign substances of course will be carried onto the wafer surface,causing damages of the wafer and resulting in yield problems. Morerecently, it was discovered by the inventor of the present inventionthat, even with ultra-pure de-ionized rinsing water, undesirableproblems can also be created. More specifically, it was discovered bythe inventor of the present invention that if abnormalities are presentin the rinsing water delivery system, the friction between water and thewater delivery system, which is typically made of dielectric materialswith high electric and corrosional resistances such as PFA, PTFE, etc,can cause electrostatic charges to be generated. These electrostaticcharges will be carried by the rinsing water onto the wafer surface.When the electrostatic charges accumulate to a certain level, they cancause a dielectric layer, on which the electrostatic charges accumulate,to rupture. This phenomenon is called “charge damage”. An operation thatcause electrostatic charges to be penetrate into the dielectric layer iscalled a charge damage.

As there exists the ever-present demand on semiconductor manufacturersto continuously reduce the physical dimensions of semiconductor devices,the resistance against charge damages that can be expected from asemiconductor device also continues to decline. As a result, it becomesincreasingly critical during semiconductor fabricating processes to beable to quickly detect the presence of electrostatic charges, perform atrouble-shooting diagnosis, and have the problems solved and processresumed, without causing interruptions in the fabricating process. Inaddition to the charges that can be carried by the rinsing water,electrostatic charge damages can also be caused by the laminar air flowin the clean room, physical touches by human bodies, high-energy ionplantations, etc. As the semiconductor manufacturers are under anenormous pressure to cut cost by improving process yields, it is equallyimportant to be able to timely and inexpensively detect thoseelectrostatic charges so as to assure a smooth operation of thefabrication process.

FIG. 1 is a flowchart diagram which shows the main steps of aconventional process for detecting the charges on a wafer surface. Itrequires, among other things, the steps of: (1) wet cleaning and forminga sacrificial oxide layer; (2) removing the sacrificial oxide layer; (3)forming a gate layer first by oxidation; (4) deposition of poly or metalon the gate layer; (5) defining the gate pattern; (6) poly or metaletching to form the gate layer; (7) damage annealing; (8) running acharge damage process; (9) gate break-down probe; and (10) discardwafers that show high charge damages. As it can be seen from FIG. 1 andthe above discussion, the conventional process for detecting the chargeson a wafer surface is time- and labor-consuming, and it causesinterruptions in the production processes.

SUMMARY OF THE INVENTION

The primary object of the present invention is to develop an improvedmethod which can be used to, cost-effectively and with only minimumprocess interruption, detect electrostatic charges following ade-ionized water rinsing step during the fabrication of semiconductordevices to remove particles and other impurities from a wafer surface.More specifically, the primary of the present invention is to develop animproved method, and an apparatus for carrying out the same, for use inthe semiconductor industry, to cost-effectively and expeditiously detectelectrostatic charges that may have accumulated on the surface of asemiconductor wafer following a rinsing step which uses pure water orde-ionized water, to remove particles, and/or other impurities. Themethod of the present invention allows high precision and highresolution scanning to be made, so that it can very quickly andaccurately pinpoint the trouble spot and allow a trouble-shootingdiagnosis to be expeditiously performed. Furthermore, the presentinvention involves a much simpler procedure than any of those in theprior art processes, and, as a result, it can be very cost-effectivelyimplemented and will not cause major interruptions in the semiconductorfabrication operation.

The method disclosed in the present invention comprises the followingmain steps:

-   -   (a) Disposing a capacitor plate above a wafer surface on which        electrostatic charges are to be measured on a local basis;    -   (b) Using a movable probe to measure voltages at various        locations at the capacitor plate;    -   (c) Collecting the measured voltage distribution at the        capacitor plate; and    -   (d) Determining area or areas on the wafer surface with        correspondingly high electrostatic charge density.

When used in a semiconductor device fabrication process, the entireprocess begins with the wafer start, followed by gate oxidation, runcharge damage process, then finally the wafer surface voltage scanningprocess described above. Finally, the gate oxide layer is removed andthe wafer is re-used for further fabrication steps.

Although the method disclosed in the present invention is mostadvantageous for use in detecting abnormal electrostatic charges afterthe semiconductor wafer has been subject to a water rinsing step, it canalso be used to provide the same function in situations where no waterrinsing is involved.

BRIEF DESCRIPTION OF THE DRAWING

The present invention will be described in detail with reference to thedrawing showing the preferred embodiment of the present invention,wherein:

FIG. 1 is a schematic flowchart diagram showing the main steps of aconventional process for detecting electrostatic charges on a wafersurface.

FIG. 2 shows a schematic drawing of the first preferred embodiment ofthe apparatus for carrying out the method disclosed in the presentinvention to detect electrostatic charges on a wafer surface.

FIG. 3 is a schematic flowchart diagram showing the main steps ofincluding the method of the present invention in a semiconductor devicefabrication process.

FIG. 4A shows results obtained from a trouble-shooting test utilizingthe method of the present invention, by measuring the voltagedistribution over the surface of a just-fabricated wafer, to solve ayield problem which was traced to a faulty nozzle.

FIG. 4B shows that the number of bit fails prior to the trouble-shootingprocedure.

FIG. 4C shows that the number of bit fails after the trouble-shootingprocedure; it shows that the number of bit fails was substantiallyreduced after the original problem was diagonalized and solved using themethod of the present invention.

FIG. 5 shows the design of a sectionalized capacitor plate used in thesecond preferred embodiment of the apparatus for carrying out the methodof the present invention to detect electrostatic charges on a wafersurface.

FIG. 6 shows a third preferred embodiment of the apparatus for carryingout the method of the present invention to detect electrostatic chargeson a wafer surface, wherein the capacitor plate has a small area whichmoves with the probe.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention discloses a novel and improved method for use inthe semiconductor industry to measure the two-dimensional distributionof electrostatic charge densities on a wafer surface following ade-ionized water rinsing step during the fabrication of semiconductordevices to remove particles and other impurities from a wafer surface.The method of the present invention allows high precision and highresolution scanning of the electrostatic charge density distribution tobe made, so that it can very quickly and accurately pinpoint the troublespot and allow a trouble-shooting diagnosis to be expeditiouslyperformed. While the method disclosed in the present invention is mostadvantageous for use in detecting abnormal electrostatic chargeaccumulations after the water rinsing step, it can also be used toprovide the same function in situations where no water rinsing isinvolved. The present invention involves a much simpler procedure thanany of those in the prior art processes, and, as a result, it can bevery cost-effectively implemented and will not cause major interruptionsin the semiconductor fabrication operation.

The method disclosed in the present invention comprises the followingmain steps:

-   -   (a) Disposing a capacitor plate above a wafer surface on which        electrostatic charges are to be measured on a local basis;    -   (b) Using a movable probe to measure voltages at various        locations at the capacitor plate;    -   (c) Collecting the measured voltage distribution at the        capacitor plate; and    -   (d) Determining area or areas on the wafer surface with        correspondingly high electrostatic charge density.

When used in a semiconductor device fabrication process, the entireprocess begins with the wafer start step, followed by gate oxidationstep, and the step of running charge damage process, then the wafersurface voltage measuring process described above. Finally, the gateoxide layer is removed and the wafer is re-used for further fabricationsteps.

The present invention will now be described more specifically withreference to the following examples. It is to be noted that thefollowing descriptions of examples, including the preferred embodimentof this invention, are presented herein for purposes of illustration anddescription, and are not intended to be exhaustive or to limit theinvention to the precise form disclosed.

EXAMPLE 1

FIG. 2 shows a first preferred embodiment of the apparatus for carryingout the method of the present invention to detect electrostatic chargeson a wafer surface.

In FIG. 2, it is shown a capacitor plate 10 which is positioned above awafer surface 11 on which a two-dimensional distribution ofelectrostatic charges is to be measured. The capacitor plate 10 iscontrolled by a mechanical vibrator 12, which causes the wafer 11 tomove vertically as well as horizontally. A movable probe 13, which isconnected to a Kelvin probe electronics 14, is utilized to measure(i.e., scanning) voltages at various locations at the capacitor plate10.

After the capacitor plate 10 is moved by the mechanical vibrator 12 to apredetermined horizontal above the wafer surface with a predetermineddistance separating the two, the movable probe 13 begins to scan overthe capacitor plate 10 to obtain a two-dimensional voltage distributionat the capacitor plate 10. The measured voltage ΔV, which is recordedwith a recorder 14, is converted to localized electrostatic charge ΔQaccording to the following equation:ΔQ=ΔC×ΔVwhere ΔC is calculated according the following formula:ΔC=ε×ΔA/dand ε is the dielectric constant of the space between the wafer surfaceand the capacitor plate 10, ΔA is the effective area covered by theprobe 13, and d is the separation between the wafer surface and thecapacitor plate 10.

After the voltage distribution is determined, the two-dimensionalelectrostatic density distribution on the wafer can be calculated.

EXAMPLE 2

FIG. 3 is a schematic flowchart diagram showing the main steps ofincluding the method of the present invention in a semiconductor devicefabrication process. Compared to the conventional process as shown inFIG. 1, the method of the present invention is much more simplified.After an oxide layer is formed (Gate Oxidation), it is subject to awater rinsing step (Run Charge Damage Process). Thereafter, the stepsdescribed in Example 1 is performed (Wafer Surface Voltage Measurement).Since the method disclosed in the present invention does not require anyadditional physical contact with the wafer surface, the waferfabrication process can be resumed without any interruption (WaferRe-use, or Gate Oxide Removal).

FIG. 4A shows results obtained from a trouble-shooting test utilizingthe method of the present invention. Initially, the accumulation ofelectrostatic charges was thought to be due to inadequate pressure ofthe de-ionized water. However, as shown in FIG. 4, increasing the waterpressure did not show any improvement. FIG. 4 shows that the pressuredistribution curves obtained at nozzle pressures of 40 psi (curve 1), 50psi (curve 2), and 60 psi (curve 3) are indiscriminate from each other.The simple and un-intrusive procedure of the method of the presentinvention allows the operator to try other options. One option was toadjust the nozzle rising angle and location. Results of this simpleadjustment were shown in Curve 4. This immediately caused theelectrostatic charge density to be substantially reduced. Furtheradjustment of the nozzle caused the problem to be eliminated. FIG. 4Bshows that the histogram of bit fails, shown as 360, of ajust-fabricated wafer prior to the implementation of thetrouble-shooting procedure described in the present invention. FIG. 4Cshows that the number of bit fails after the trouble-shooting procedure;it shows that the number of bit fails was substantially reduced afterthe original problem was diagonalized and solved using the method of thepresent invention. The effectiveness is particular profound in thecenter region of the wafer 370.

EXAMPLE 3

In the second embodiment of the present invention, the apparatus isidentical to that described in Example 1, except that the capacitorplate 10 is made of a plurality of isolated electrode sub-plates 20.This embodiment allows a better defined capacitor area to be employedfor more quantitative results.

EXAMPLE 4

In the third embodiment of the present invention, the apparatus isidentical to that described in Example 1, except that the capacitorplate 30 has a very small area and it moves with the probe. Thisembodiment also allows a better defined capacitor area to be employedfor more quantitative results.

The foregoing description of the preferred embodiments of this inventionhas been presented for purposes of illustration and description. Obviousmodifications or variations are possible in light of the above teaching.The embodiments were chosen and described to provide the bestillustration of the principles of this invention and its practicalapplication to thereby enable those skilled in the art to utilize theinvention in various embodiments and with various modifications as aresuited to the particular use contemplated. All such modifications andvariations are within the scope of the present invention as determinedby the appended claims when interpreted in accordance with the breadthto which they are fairly, legally, and equitably entitled.

1. A method for detecting electrostatic charges on a wafer surface,comprising the steps of: (a) disposing a capacitor plate above a wafersurface on which electrostatic charges are to be scanned; (b) using amovable probe to measure voltages at various locations at the capacitorplate; (c) collecting the measured voltage distribution; and (d)examining the collected voltage distribution to identify areas on thewafer surface correspondingly to high electrostatic charge density. 2.The method for detecting electrostatic charges on a wafer surfaceaccording to claim 1, wherein the wafer contains a dielectric layer atits outmost surface.
 3. The method for detecting electrostatic chargeson a wafer surface according to claim 2, wherein the dielectric layer isan oxide layer.
 4. The method for detecting electrostatic charges on awafer surface according to claim 1, wherein the method is performedfollowing a cleansing step using pure water or de-ionized water toremove particles or other impurities on the wafer surface.
 5. The methodfor detecting electrostatic charges on a wafer surface according toclaim 1, wherein the capacitor plate is structured such that it can bemoved both vertically and horizontally above the wafer surface.
 6. Themethod for detecting electrostatic charges on a wafer surface accordingto claim 1, wherein the capacitor plate is made of a plurality ofcapacitor sub-plates electrically insulated from each other.
 7. A methodfor detecting electrostatic charges on a wafer surface, comprising thesteps of: (a) disposing a capacitor plate above a wafer surface on whichelectrostatic charges are to be scanned; (b) attaching a probe on thecapacitor plate; (c) moving the capacitor plate horizontally above thewafer surface so as to allow the probe to measure voltages at variouslocations above the wafer surface; (d) collecting the measured voltagedistribution; and (e) examining the collected voltage distribution toidentify areas on the wafer surface correspondingly to highelectrostatic charge density.
 8. The method for detecting electrostaticcharges on a wafer surface according to claim 7, wherein the wafercontains a dielectric layer at its outmost surface.
 9. The method fordetecting electrostatic charges on a wafer surface according to claim 8,wherein the dielectric layer is an oxide layer.
 10. The method fordetecting electrostatic charges on a wafer surface according to claim 7,wherein the method is performed following a cleansing step using purewater or de-ionized water to remove particles or other impurities on thewafer surface.
 11. The method for detecting electrostatic charges on awafer surface according to claim 7, wherein the capacitor plate isstructured such that it can be moved both vertically and horizontallyabove the wafer surface.
 12. An apparatus method for detectingelectrostatic charges on a wafer surface, comprising the steps of: (a)movable capacitor plate to be placed above a wafer surface on whichelectrostatic charges are to be scanned; (b) a movable probe to measurevoltages at various locations at the capacitor plate; and (c) a recorderto collect and record the measured voltage distribution.
 13. Theapparatus for detecting electrostatic charges on a wafer surfaceaccording to claim 12, wherein the wafer contains a dielectric layer atits outmost surface.
 14. The apparatus for detecting electrostaticcharges on a wafer surface according to claim 13, wherein the dielectriclayer is an oxide layer.
 15. The apparatus for detecting electrostaticcharges on a wafer surface according to claim 12, which is to beperformed following a cleansing step using pure water or de-ionizedwater to remove particles or other impurities on the wafer surface. 16.The apparatus for detecting electrostatic charges on a wafer surfaceaccording to claim 12, wherein the capacitor plate is structured suchthat it can be moved both vertically and horizontally above the wafersurface.
 17. The apparatus for detecting electrostatic charges on awafer surface according to claim 12, wherein the capacitor plate is madeof a plurality of capacitor sub-plates electrically insulated from eachother.